31
The numbers in the sequence are
13 15 21 27
They can be expressed as the prime numbers [11 13 17 19 23] added to [2 2 4 4 4]. If we add the next sequential prime number to the first matrix, it would be 27. Of the second matrix, two repeats itself twice, so four should repeat itself four times, so let's add four to it. 27+4=31, the next number in the sequence.
I'm probably making this problem waaaay more complicated than it should be, but still...
